I. Field of the Invention
The present invention relates generally to the treatment of wastewater, and other liquid waste streams, and more particularly to the removal of proteinaceous material and suspended solids from wastewater using only carbon dioxide as a combination of treatment chemical, separation method and anti-biological agent in a manner that conserves and reuses excess carbon dioxide efficiently.
II. Description of the Problem
Businesses engaged in the processing of food and other organic substances, such as poultry processing facilities, generate tremendous quantities of wastewater containing a wide range of dissolved organic fluids and suspended solids. In poultry processing operations, chickens are brought to the facility alive where they are hung upside down, have their throats slit to drain away blood, are scalded to remove feathers, then the internal organs removed, followed by plunging the carcass into a chilled water bath before further processing. Thus, contaminants are added to wastewater stream that may be suspended particles, such as feather pieces, emulsified and partially soluble materials like fats and oils, and dissolved materials such as blood proteins. For the purposes of this disclosure, the blood and other aforementioned biological matter containing proteins will be collectively referred to herein as "proteinaceous material." During further processing, other particulates such as marinades, oils, fats, breading, batter and seasonings, migrate to the wastewater stream. These substances may come from frying, grilling, roasting, steam cooking, impingement cooking, chilling, freezing and various material handling steps. In many instances, it is through the periodic sanitation phase when machinery is cleaned that many suspended solids enter the wastewater stream.
Another example of a facility wherein such wastewater is generated is in a fully-cooked poultry steam application, where 10,000 gallons per day of liquid enters the waste stream in a typical application. This equates to approximately 80,000 pounds per day at a ten percent (10%) solids content. Currently this level of solids enters the waste stream and places high demand on the treatment system.
The industry attempts to recover as much material as possible in each processing step for reuse by rendering it back into animal feed products. However, since water is a crucial component to the overall process, it invariably contains large amounts of material that must be removed prior to discharge to the environment. Indeed, in recent years, the rapid growth of the poultry processing industry has resulted in large volumes of water being generated in such processing facilities. Pressure is being brought to bear on the industry to suitably clean this water prior to discharge. New technologies, such as the one disclosed herein, may offer the potential to salvage edible materials for human or animal consumption at the point of application, thus resulting in overall decreases in water consumption and treatment chemicals.
The problems associated with the removal of such proteinaceous materials and suspended solids in wastewater streams are well known. Various techniques have been proposed and tried to treat the water with different degrees of success and associated costs. The most common technique has been to perform some mechanical treatment to remove coarse suspended solids and subject the water to both anaerobic and aerobic biological treatment. This technique is often chosen because of its relatively low capital cost. However, most processing facilities of this type make use of large amounts of disinfectants and hot water, which has a profound impact on the required biological processes that is difficult to control.
Because the chicken fecal material contains an abundance of sulfur-reducing bacteria, and because blood protein is also high in sulfur content, the anaerobic ponds typically used as a first step produce hydrogen sulfide gas and other odor compounds that are objectionable to the surrounding area. Furthermore, the anaerobic ponds do not efficiently process the waste and subsequently lose their effectiveness. Since these ponds are normally followed by aerobic treatment, their ineffectiveness results in increased contaminant loads on the aerobic process.
The increased loads on the aerobic system ultimately lead to a failure ofthe aerobic treatment to effectively treat the wastewater. Unfortunately, the poultry industry is faced with increasing the capacity of these facilities in the face of ever more stringent discharge limitations. Since these facilities all require significant amounts of land area, many are limited as to expansion options. Those without available land area are faced with using physical or chemical treatment techniques to try and achieve discharge water quality.
For example, the poultry industry currently employs the addition of chemicals to coagulate the suspended solids, as well as some type of clarification equipment to remove this material. As is common in this particular industry, some dissolved materials require a lowering of the pH of the stream to a point where optimum chemical reactions may proceed to cause the dissolved material to precipitate from the stream in the form of a suspended solid. This is particularly true in the case of blood products mixed into the waste stream. In order to accomplish this, the pH of the waste stream must be reduced to below about pH 4.5 at which level the blood proteins coagulate from solution. This is generally accomplished with an inexpensive inorganic acid, such as sulfuric or hydrochloric acid. In many instances, the pH of the stream and separated solids, after separation of the suspended solids from the stream, is below that pH limit suitable for reuse or discharge to another process or to the environment. When unacceptably low pH conditions exist, the pH of the treated stream or the separated solids must be increased to acceptable levels.
To return the pH to acceptable levels, a base material, again usually inorganic, is added which reacts with the acid. There are several problems associated with chemical treatment processes such as that just described, especially if the acidic pH level is below about pH 4.5. First, although the addition of the acid causes the pH to decrease, it may also bring about an interaction with the suspended and dissolved materials, which consumes the acid and which may create undesirable reaction products. Second, the precipitated material resulting from the reduction in pH will also be acidic in nature and, after separation, may be unsuitable for any reuse unless treated to return the pH to higher levels. Third, the addition of a base to either the liquid stream or the separated solids results in the formation of salts which may not be acceptable in reuse of the solids or the treated liquid. Also, the sedimentation clarifiers typically used in connection with chemical treatment techniques are ineffective in removing oils and grease. Therefore, most designers opt for some type of flotation clarifier as explained further below.
As an alternative to chemical treatment using acids, some facilities have turned to flotation clarifier processes whereby air is injected into the waste stream under pressure. At the appropriate time, the pressure within the chamber containing the wastewater is reduced, producing fine bubbles which adhere to the solids and cause them to float to the surface. The floating solids are then removed by conventional skimming equipment for later use or disposal. However, while those processes involving dissolved air do eliminate some of the problems of chemical treatment, other problems still remain. For example, the added air may promote the growth of bacteria in the separated solids, thereby causing odors and degradation of the material. As a result, the recovered solids may be unfit for reuse, especially in the case of such materials from food processing operations.
An example of physical and chemical treatment is U.S. Pat. No. 5,413,720 issued to Miller. Miller describes what may be considered the current industry practice for treating the wastewater, particularly in poultry operations, in an attempt to recover the material in a form suitable for reuse in food. While the Miller patent does make an effort to use chemical additives that are considered safe in animal food, it still relies on mineral acids to reduce the pH. Therefore, that process must contend with all the problems described previously herein in connection with the addition of acid.
For example, in chemical processes such as that described in Miller, the amount of acid needed to effect the pH reduction, together with the added polymers need to further coagulate all the suspended materials, adds an appreciable cost to the process. This is further complicated by the fact that the mineral acids degrade the proteins, as well as the polymers themselves to some extent, via oxidation. The resulting solids are increased by up to 35% in weight and exist in a partially degraded and highly acidic form, thereby making them generally unsuitable for rendering into animal foods. To further complicate matters, these solids must be neutralized with a base material before being made suitable to disposal into land applications. Also, the water, after clarification, must also be neutralized with a base material, usually sodium hydroxide or lime, to raise the pH to suitable values for discharge. This step not only adds more cost, but also results in even greater weight and volume to the solids for disposal, as well as additional dissolved salts in the water.
The following is a brief discussion of several patents which describe various prior art processes. U.S. Pat. No. 4,267,050, issued to Conway, et aL, relates to a process for removing solids from wastewater including dissolving gas having a solubility in water higher than that of air, preferably carbon dioxide, into an aqueous fluid stream at a pressure higher than a flotation zone in order to cause the gas to dissolve in the stream, and then expanding the pressurized stream into a flotation zone where pressure is released causing bubbles to form and adhere to insoluble impurities and thereby cause the contents to separate by flotation. The preferred source of carbon dioxide is from burning methane gas. Conceptually, a reduction in capital and operating costs are derived from the higher solubility of the carbon dioxide in water versus air and utilizing combustion gases as the source of the carbon dioxide which is presumed to be economical over purchase of pure carbon dioxide. No mention is made of reducing the pH of the wastewater stream by an amount sufficient to cause coagulation of blood proteins and other proteinaceous materials.
U.S. Pat. No. 4,289,628, issued to Disselbeck, et al., is drawn to a process for separating solids from liquids in food processing, poultry farms and the like using air without the use of flocculating agents but with a particular type of filter media. While the patent mentions the use of carbon dioxide as alternative to air, no mention is made of any manner by which the carbon dioxide is introduced into the system, nor what purpose it would serve other than as a flotation aid. A closed system is recommended for use with nitrogen or carbon dioxide, but only to the extent as to exclude oxygen and for no other purpose.
U.S. Pat. No. 2,350,111, issued to Hood, relates to clarification of waste liquids, and teaches the desirability of an initial introduction of carbon dioxide into the untreated liquid, rather than strong acids (which, it is taught, may be undependable and have a bactericidal effect). However, subsequently in the process of Hood, a chemical electrolyte is used for coagulation and sedimentation. It is apparent that the invention of Hood lacks the ability to introduce carbon dioxide in sufficient quantity to reach the isoelectric point of proteins that may be present.
U.S. Pat. No. 4,217,217, issued to Kay, et al., teaches treatment of alkaline waste water by introducing therein finely divided carbon dioxide by using diffusers. The focus of Kay was to use the carbon dioxide primarily to neutralize the alkaline wastewater, and to design the diffuser system and tank to utilize most of the carbon dioxide so that very little is wasted. There is no mention of reducing the pH of the wastewater stream by an amount sufficient to cause coagulation of proteinaceous materials.
U.S. Pat. No. 4,137,163, issued to Young, is directed to a process for treatment of waste streams which includes scrubbing hot combustion gas (containing carbon dioxide) with the effluent so as to produce a foam. For example, it is disclosed that the reducing atmosphere of the flue gas improves flocculation. This patent centers around using combustion gas to reduce the cost of carbon dioxide, and it does not suggest the lowering of the pH to cause coagulation of waste constituents.
U.S. Pat. No. 3,419,493, issued to Pangle, is drawn to a process for treating textile mill waste water, which includes bubbling through the wastewater supernatant from the floc and settling zone, a gas which contains carbon dioxide, in order to adjust the pH and form a foam. The idea is to use carbon dioxide in conjunction with air for the production of a foam to cause material to float.
U.S. Pat. No. 5,167,806, issued to Wang, describes avery complex system of compressor, pump and high speed, rotating tank with specific internals for dissolving and releasing gases in liquids. The focus of this patent is concerned with the high speed, rotating tank for contacting the gases with the liquid and its use in biological systems.
Although the foregoing description of the problems of wastewater treatment have been explained in connection with poultry processing facilities, such facilities are only a small part ofthe many applications of wastewater treatment involving physical and chemical treatment where pH lowering is needed, followed by increasing the pH to acceptable levels after clarification. There exists, therefore, a great need for a process to effectively treat these wastewater streams in an economical fashion that eliminates the use of mineral acids and bases, while minimizing the use of other chemicals, and which excludes the introduction of air into the separation process.